This invention relates to superconducting magnets. More particularly, the invention relates to cryogen pressure vessel assemblies for superconducting magnets.
As is well known, a coiled magnet, if wound with wire possessing certain characteristics, can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold reduces the resistance in the magnet coils to negligible levels, such that when a power source is initially connected to the coil to introduce a current flow through the coils, the current will continue to flow through the coils due to the negligible resistance even after power is removed, thereby maintaining a magnetic field. Superconducting magnets find wide application, for example, in the field of magnetic resonance imaging (hereinafter “MRI”).
A typical superconducting magnet assembly for use in MRI consists of two sets of superconducting coils disposed in a cryogen vessel. An inner set of coils, usually called the main magnet coils, produce a uniform magnetic field of large magnitude in an imaging volume. The conventional cryogen pressure vessel is a circular, cylindrical drum in which a liquid cryogen, such as helium, is maintained under pressure. The main coils are wound separately around coil formers, also known as spools or bobbins, placed in grooves machined in the drum and spaced axially along the inside of the drum. Another set of outer magnet coils, known as bucking coils, are spaced from and surround the main coils, and are supported by a structure secured to the drum. The bucking coils carry currents in the direction opposite to the direction of currents being carried by the main coils so as to cancel the stray magnetic field outside the magnet.
An example of an MRI system including a superconducting magnet assembly is described in European Patent No. EP 0 587 423. As shown in FIG. 1, the magnet assembly includes a superconducting main magnet field coil assembly 10 which generates a substantially uniform magnetic field longitudinally through an examination region 12. A gradient magnetic field coil assembly 14 selectively creates gradient magnetic fields across the examination region 12. A gradient magnetic field control means 16 controls a current pulse generator 18 to apply current pulses with selected characteristics to the gradient field coils to cause the desired magnetic field pulse to be generated.
A resonance excitation and manipulation means includes a radio frequency transmitter 20 for generating radio frequency pulses of the appropriate frequency and spectrum for inducing resonance of selected dipoles in the examination region 12. The radio frequency transmitter is connected with a radio frequency antenna 22 disposed surrounding the examination region and inside the gradient magnetic field coil assembly 14. The RF coil transmits radio frequency pulses into the region of interest and receives radio frequency resonance signals emanating therefrom. Alternatively, a separate receiving coil may be provided. The received magnetic resonance signals are conveyed to a digital radio frequency receiver 24 for demodulation. The demodulated, digital radio frequency signals are reconstructed into a magnetic resonance image representation by an array processor or other image reconstruction means 26. The reconstructed image representation is stored in an image memory 28. The image representation may be displayed on a video monitor 30, subject to further processing, stored on tape or disk, or the like.
The superconducting magnet assembly 10 includes an outer vacuum vessel 40 which defines an inner, cylindrical room temperature bore 42 within which the gradient field coil assembly 14 is received. A series of superconducting, annular magnetic coils 44 are mounted on a coil former 46 and disposed within an annular cryogen pressure vessel 48. A port 50 permits the cryogen pressure vessel 48 to be maintained filled with liquid helium or the like as it evaporates to hold the temperature within the pressure vessel at about 4.2° K. Preferably, a helium recovery and recondensing system (not shown) is interconnected with the port 50. Also disposed within the cryogen pressure vessel 48 is a bucking coil assembly 56, which is mounted around the exterior of the superconducting magnet coils 44 and connected electrically in series therewith. The bucking coil assembly 56 generates a magnetic field which opposes the fields generated by the main magnets 44 in the exterior of the cryostat, while producing a strong uniform magnetic field along the bore 42. The bucking coil assembly comprises magnetic coils 58 wound around a coil former 62. The cryogen pressure vessel 48 is surrounded by a first cold shield 52 which is cooled to about 200 K. or less. A second cold shield assembly 54, which is chilled to about 60°-70° K. or less, is disposed between the inner cold shield assembly and the vacuum vessel 40. In this way, a series of thermal gradations are maintained to minimize the evaporation of the cryogen.
There are many factors that challenge the designer of a superconducting magnet assembly. First, the assembly is subject to many stresses. For example, in the process of energizing the magnets, the coils and coil former are subjected to significant electromagnetic loading. In the process of cooling the coils to superconductive temperatures, uneven cooling rates and the use of a mix of materials in the coil former can cause differential expansion in the coils and coil former, which creates stresses in the coils and coil former. The relief these stresses can cause the sudden movement of the coils, which is a major cause of quenches (rapid loss of superconductivity and collapse of magnet field) in the magnets. In addition, relative movement of coils as a result of small changes in ambient temperature or pressure can also cause inhomogeneity in the magnet. Second, space is at a premium, with many modern designs aiming to make the magnet as small as possible. These and many other factors combine to make the design of a superconducting magnet assembly very challenging.